Black/White 2 and HGSS would like a word with your never good comment, and even the likes of sword and shield have pretty decent postgames with the dlcs
When it comes to the post games in Pokémon, I am inclined to strongly agree with you. In my opinion, the post-game plays a huge part in what makes Pokémon games great. The first games, generation I, that took place in Kanto did not have much of a post-game, but this should be expected because the series began with these games. Series mainly add new features as more titles are released. The generation II games that took place in Johto had a great post game. Being able to return to Kanto and conquer the gyms again before taking on Red, the other game's protagonist, was a
great post game and established an identity for generation II. I honestly don't remember generation III that much because I only played it once a long time ago, but I do recall generation III having excellent battle facility features for the post game. As for generation IV, I don't remember the post-game for Diamond, Pearl, and Plantium too well. If you can recall them, please refresh my memory. I personally wasn't a fan of Sinnoh, so I just wanted to finish the game quickly as possible, considering that I hated the HM slavery and fog.
However, HeartGold and SoulSilver definitely had an amazing post-game as you said. They expanded on the already great post-game that existed in the second generation. Generation V, especially the second games, Pokémon Black 2 and White 2, had great post-games as well, as well as great battle facilities. Along with the generation II remakes, I must say that I loved these games for their post-game content as well. The games that took place in Kalos also had a great post game in my opinion. The story with Looker and the homeless girl Emma was very emotionally moving for me and I enjoyed it a lot. Generation 7, the games that took place in Alola, had great post games. The first games, Sun and Moon, had fun quests where you track the Ultra Beasts for Looker. The generation III remakes, Omega Ruby and Alpha Sapphire had great post games too. You can't tell me that you didn't have fun soaring through the sky and hunting those legendaries from previous games. I know I did! Ultra Sun and Ultra Moon had you fight Team (Rainbow) Rocket again for old times sake during the post-game.
This time, they took over Festival Plaza, and brought along every other villain team that ever existed in the series with them. The bosses of each villain team had their game's box legendary on their teams and Giovanni had Mega Mewtwo. It was super lit. I loved having the chance to beat them all again and face the legendaries, especially Ghetsis again. Good times. Of course, generation 8, the Galar games, brought us the storyline with the weird hair brothers, which I thought was weak to be honest, but I loved the DLC. With that being said, I strongly agree with you. Post-game content is a big part of Pokémon. However, I don't think it is experiencing a downfall, so to speak. If anything, the developers are simply using DLC as a post-game and using it as a cash cow these days instead of content that should already be included in the game. In my opinion, Scarlet and Violet could definitely use some more post-game content and I hope it's coming soon.